IMPACTS OF LARGER, MORE SEVERE AND FREQUENT WILDFIRES ON GEOMORPHIC PROCESSES AND LANDSCAPE EVOLUTION IN ARIZONA

Increasingly large, severe and frequent wildfires occurring in Arizona are altering watershed hydrology and sediment production, resulting in increased hazards to downstream development. Vastly increased runoff from steep watersheds with moderate to high severity burned areas leads to erosion of sediment from gullies and channels, and the generation of debris flows and floods. Postfire debris flows are typically generated by frequently occurring (1+ yr recurrence interval (RI)) monsoon rainfall events. Postfire floods are common and pose significant risks to downstream communities. A 2003 postfire flood in the Santa Catalina Mtns near Tucson, generated from a 10-yr RI rainfall, had a peak discharge near the envelope curve for lower Colorado River watersheds, demonstrating the magnitude of hydrologic changes in burned watersheds. In 2010 a ~20-yr RI storm over the Schultz Fire near Flagstaff resulted in numerous debris flows that eroded swales up to 4 m deep, and floods that severely impacted downstream development. Because debris flows perform more work shaping channels and moving coarse material than the combined effects of hundreds of years of water floods, they are critical agents in shaping the watersheds of Arizona. Debris flows typically excavate nearly all sediment from steep channel reaches, suggesting that the time to reload channels with enough sediment to generate another debris flow might be centuries. The increasing frequency of wildfire occurrence is altering this situation and drastically decreasing the return time between debris flows in some watersheds, however. Areas in the Huachuca Mtns of southeastern Arizona that were burned in the 1977 Carr or 1988 Peak Fires were reburned in the 2011 Monument Fire. Watersheds that had debris flows in 1977 once again had debris flows in 2011, implying that with fairly frequent large wildfires, the return time between debris flows may be much shorter. In the Chiricahua Mtns, a ~10 m deep gully formed in one or more debris flows immediately after the 1994 Rattlesnake Fire. By 2004 the gully had filled to a depth of about 3-4 m. The area was reburned in the 2011 Horseshoe 2 Fire and the gully was re-scoured to as much as 12 m deep, implying that a geomorphic threshold may have been crossed and that gullying and debris-flow generation is likely to continue after new fires.